1. Field of the Invention
The present invention relates generally to reconstruction of signals that were encoded using pulse code modulation (PCM) or differential pulse code modulation (DPCM) techniques and, in particular, to reconstruction of such signals which have been quantized.
2. Description of the Prior Art
In most communication systems that employ any type of digital encoding (e.g., pulse code modulation (PCM) and differential pulse code modulation (DPCM) systems), samples of the signal being processed are quantized before being encoded and transmitted to the receiver. Quantizers in PCM encoders assign a particular one of `N` representative values to each sample, depending upon which of a corresponding `N` mutually exclusive quantization levels brackets that sample value. In a DPCM encoder, the quantizer assigns a representative value to the prediction error formed by subtracting a predicted version of each sample from its true value. To increase transmission efficiency, the representative values output from the quantizer are typically encoded with unique code words, such that more frequently occurring representative values are encoded with shorter words. At the receiver, each code word is decoded by assigning the appropriate representative value (hereafter sometimes referred to as "normal representative value") to the quantized quantity i.e., sample value in PCM or error value in DPCM. As stated previously, the normal representative value for each received code word is typically selected within the range defined by the upper and lower limits of the particular encoder quantization level into which the quantized quantity fell. The selection of quantizer representative values has traditionally been determined by considerations of minimum mean-square quantization error.
From the foregoing description, it is seen that in conventional PCM systems, no use is made of the spatial or temporal correlation of neighboring samples, either at the transmitter for coding, or at the receiver for reproduction. In DPCM systems, previously transmitted values are typically used for coding at the transmitter. However, little or no use of correlation has been made at the receiver for reproduction. This essentially "wastes" information available at the receiver and results in a larger reconstruction error. In addition, use has not been made of the fact that the range of quantizer input values defined by the upper and lower limits of the different quantizer levels is a known quantizer characteristic.